Vehicular and other suspension systems customarily include suspension units each comprised of a spring and a parallel damper or shock absorber that interconnect relatively movable vehicle components such as the vehicle body and frame. The damper normally is a passive one which is not connected to a pump or other high pressure source of hydraulic fluid and which generates damping forces only in response to, and in opposition to, relative movement between the interconnected vehicle components. A recognized deficiency of a passive damper is that under certain conditions of relative movement of the interconnected vehicle components, the damper will amplify rather than attenuate motion of the component that is to be isolated from shocks, vibrations and similar disturbance inputs.
The foregoing undesirable characteristic of passive dampers is not shared by fully active dampers. These employ hydraulic pumps, or other sources of pressurized hydraulic fluid, and associated control components by which the magnitude and direction of the forces generated by the dampers may be so regulated as to always effect the desired motion attenuation irrespective of the condition of relative movement between the interconnected vehicle components. Due to their inclusion of a hydraulic pump or similar pressurized fluid source, however, systems utilizing fully active dampers are relatively heavy and bulky, and are expensive from the viewpoint of both their acquisition and operating costs.
Although less conventional, dampers of the so-called "semiactive" type are also known. A semiactive damper, in keeping with a purely passive one, is not connected to a hydraulic pump or other source of pressurized fluid, and can only generate damping forces in response to and in opposition to relative movement between the vehicle components interconnected by it. Unlike a passive damper, however, a semiactive damper has control means for rapidly varying in a controlled manner the damping coefficient of the damper. This permits the damping forces generated by the damper to be of different magnitude at desired times. A simple form of a semiactive damper consists of a hydraulic piston-and-cylinder assembly having adjustable valve means within a passageway interconnecting opposite end portions of the cylinder of the assembly. The valve means throttles, to a greater or lesser extent, the fluid flow induced within such passageway by extension and retraction of the assembly as a result of relative movement between the vehicle components interconnected by it. When the relative motion condition between the vehicle components and across the damper is such that damping forces would attenuate undesired motion of the vehicle component whose motion is to be minimized, the valve means is caused to restrict or throttle the fluid flow. This increases the damping coefficient of the damper and results in the generation of damping forces of significant magnitude. On the other hand, when the relative motion condition across the damper is such that the generation of damping forces would amplify rather than attenuate motion of the vehicle component, the valve is fully opened so as to reduce the damping coefficient and cause the damping forces to be of reduced (ideally zero) magnitude. Although a semiactive damper cannot fully achieve the same degree of performance as an active damper, it can when properly controlled closely approach such performance for considerably less cost.
In the case of a vehicle suspension or other system having a semiactive damper and only a single degree of freedom, good performance can be realized when the control valve of the damper is adjusted in accordance with a relatively simple control policy such as disclosed in U.S. Pat. Nos. 3,807,678 and 4,491,207. However, many vehicle suspension and other systems are complicated high order, nonlinear systems having multiple degrees of freedom and a plurality of dampers. Such systems are customarily subjected to a variety of forces and effects of the "disturbance" type that cannot be directly modulated or controlled. For example, the suspension system of an automobile or similar vehicle is subjected to disturbance inputs produced by irregularities in the road surface over which the vehicle travels, by windage, by operation of the vehicle's engine or its powered accessories, and by the response of various of the vehicle's components to inertia forces occurring during acceleration, deceleration, cornering, etc. Optimal performance of a complex system of the foregoing type cannot be achieved readily, if at all, by simple control policies successfully employable for semiactive damper control in less complex systems.
Systems employing fully active (as opposed to only semiactive) dampers have heretofore been controlled by feedback control means that monitor one or more system parameters to determine actual system performance, which is then compared with a predetermined performance standard. The difference between the actual system performance and the predetermined standard is used to modulate the force input to, and therefore the force output of, the active dampers in a manner tending to eliminate the aforesaid difference. The use of similar control techniques in association with a complex suspension system utilizing semiactive dampers, instead of active ones, is much less likely to result in optimal system performance. This is due at least in part to the absence in a semiactive system of a hydraulic pump or other source of pressurized hydraulic fluid which can introduce forces of substantially any desired magnitude and/or direction into the system at each of the dampers independently of the then existing conditions of relative movement across the dampers. The inability of semiactive dampers to generate damping forces except in response to and in opposition to relative movement thereacross significantly increases the difficulty of achieving optimum system control by ordinary feed-back control techniques.
A more recently developed type of plant control means uses an estimator or "observer" to model the controlled system or plant. However, in most if not all instances the observer is used only for a limited purpose, such as estimation of system inputs or outputs not readily capable of direct measurement, and not for overall system management.